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Himalayan Geology, Vol. 31
(2), 2010, pp. 145-166, Printed in India
Mapping of 3-D varying structures having variable surface curvatures. I.
Feasibility of using full, Multimode-Multistructure Procedures in the
extended Himalaya
G. GURUNG1, F. SCHWAB2, B.G. JO1
1Dept. of Earth and Environmental Science, Chonbuk National University,
Jeonju, Republic of Korea 561-756
2P.O. Box 241145, Los Angeles, California 90024-1145, USA
Abstract: Modern computational hardware and Internet (network)
communications have led to an advanced multimode seismic procedure for 3 D
mapping of realistic structures. The four-part method involves: construction
of an initial 3 D structure, static computations (the feasibility aspects of
which are treated here), wavefront-propagation computations (theoretical
seismograms), and inversion for an improved structure. The static
computations assign a full, azimuthally-dependent, propagating-mode (spheroidal
and torsional) specification to each latitude-longitude location of the
geographical region. The fundamental assumption for modal treatment of a 3 D
varying structure with variable curvature, is that each triplet (frequency,
mode number, surface azimuthal direction of propagation) at a location can
be assigned its own specific laterally-homogeneous structure and radius of
surface curvature. The extent of the true structure used for this is defined
by the modal depth of penetration and the vertical cylinder with diameter
equal to 1.5 times this depth of penetration. Within the assumption, the
extended Himalaya is used to test quantitatively the feasibility of the
mapping procedures: (1) relative to required computation time and storage
for 10 km lateral resolution, a 10 10 km grid of surface locations,
surface azimuths 0.00(11.25)348.75at each location, surface dimensions of
about 2500 3100 km, depths from surface to 800 km, and frequencies
0.0005(0.0005)0.1000 Hz. With the workstation, computer technology of 2008
the static computations treat the 77,900 locations in roughly 3 months with
30 nodes and 866 Gb of disk storage per node. In the region about a single
latitude-longitude location, feasibility tests then show that: (2) relative
to treating lateral heterogeneity, both torsional and spheroidal Modes 1-13
are effective in resolving the structural parameters if the experimental
data are accurate to 3.0 significant figures (e.g. an error of 0.02-0.03
km/sec in measured phase velocity), and for experimental accuracy of 3.7
sig. fig. (a 0.005-0.010 km/sec error), almost all parts of all possible 32
modes can be used; (3) relative to treatment of variable surface curvature,
if the experimental data are more accurate than 2.4 sig. fig. (errors less
than 0.03-0.08 km/sec), the correct mode-frequency dependent radius of
curvature must be used in computations; (4) relative to treatment of modal
propagation in varying surface azimuthal directions, if the experimental
data are more accurate than 1.7 sig. fig. (errors less than 0.20-0.60
km/sec), computations must be performed as a function of azimuth for each
location.
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